The present invention relates generally to power control circuitry, and more particularly relates to techniques for improving output power control in an amplifier circuit.
In wireless transmission systems, amplifier circuits may be employed, for example, in a transmitter section of a network interface card (NIC). Such amplifier circuits are generally required to provide, among other characteristics, certain guarantees of linearity and output power level control in order to insure interoperability among NICs of various manufacturers. In order to provide such guarantees, conventional wireless local area networks (LANs) are typically designed according to a specified standard, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. This standard, as set forth in the document IEEE Std. 802.11 (ISO/IEC DIS 8802-11), entitled Supplement to IEEE Standard for Information Technologyxe2x80x94Telecommunications and Information Exchange Between Systemsxe2x80x94Local Metropolitan Area Networksxe2x80x94Specific Requirementsxe2x80x94Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 1999 Edition, which is incorporated herein by reference, describes, among other things, the radio frequency (RF) range, data speed, modulation and acceptable power levels of the output signal of the NIC.
In order to meet the requirements set forth in the IEEE 802.11 standard, and in the supplements relating thereto, conventional power amplifier circuits generally utilize some form of power control loop (PCL) to help maintain a relatively constant output signal from an RF power amplifier (PA) in the NIC to an antenna, regardless of variations in, for example, input signal, output signal (e.g., as a consequence of additional circuits between the output of the RF PA and an RF sensor), antenna mismatch (e.g., as a consequence of antenna position relative to certain objects), voltage supply and temperature. According to IEEE 802.11, for binary phase shift keying (BPSK) modulation, a data rate of one megabit per second (Mb/s) is supported. For quadrature phase shift keying (QPSK) modulation, IEEE 802.11 supports data rates of 2 Mb/s, 5.5 Mb/s and 11 Mb/s.
Both the BPSK and QPSK modulation techniques result in a relatively constant envelope of the modulated signal. However, as the data rate increases (e.g., 54 Mb/s) the 1 decibel (dB) compression point, a measure of the linearity of an active component, increases significantly beyond an average power of the amplifier. For example, for low data rate BPSK or QPSK, the 1 dB compression point of the PA must be approximately 3 to 4 dB above the average power in order to guarantee sufficient linearity for errorless transmissions. For a 64-bit quadrature amplitude modulation (64-QAM) technique, the peak-to-average power (PAP) ratio of the modulated carrier is approximately 10 dB. For a PA that generates a 15 dBm average power level (where dBm may be defined as a unit for expression of power level in dB with a reference to a power of one milliwatt), the 1 dB compression point will be at approximately 25 dBm output power in order to guarantee sufficient linearity.
There is a need, therefore, for improved power control techniques for controlling an output power level of an amplifier circuit so as to guarantee a substantially constant output signal of the amplifier circuit, with sufficient linearity.
The present invention provides techniques for controlling an output power level in an amplifier circuit such that an envelope of the output signal is held substantially constant while providing a desired linearity in the output signal. The power control techniques of the present invention are particularly well-suited for use in a network interface card application constructed in accordance with, for example, the IEEE 802.11 standard.
In accordance with one aspect of the invention, an amplifier circuit having a controllable output power level comprises an attenuator configurable for receiving an input signal and operative to control an attenuation thereof, and an amplifier coupled to the attenuator, the amplifier receiving the attenuated input signal and generating an amplified output signal. The amplifier circuit further includes at least one power detector configurable to independently detect at least a portion of an incidental wave and a reflected wave corresponding to the amplified output signal and to generate first and second signals representative of the detected incidental and reflected waves, respectively. A controller is coupled between the power detector and the attenuator in a feedback configuration, the controller controlling the attenuation of the input signal and/or an amplification of the input signal as a function of the first and second signals from the power detector. In this manner, the output signal of the amplifier circuit is substantially maintained at a predetermined power level.
These and other features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.